Plug valves are used in the oil and gas industry for heating and treating systems, cementing, fracturing and acidizing and other well stimulation applications in which high working pressures and abrasive and/or corrosive fluids are conducted at high flow rates.
Prior art plug valves are subject to wear. However, attempts have been made to permit at least some components in the flow path to be replaced. For example, U.S. Pat. No. 6,675,825 to Reeves et al., includes a valve body, an insert (or “cage”) fitted non-rotatably within the valve body and a plug rotatable within a cylindrical cavity in the cage between an open position, in which fluid flows through the valve, and a closed position, in which fluid flow is obstructed. In the open position, fluid flows through a bore through the plug. In the closed position, the bore in the plug is rotated perpendicular to the flow path and the plug blocks the flow path. When the cage and/or the plug become worn with use, one or both can be replaced without replacing the expensive valve body.
In use, a plug valve is frequently subjected to high pressures, corrosive fluids and abrasive proppants (such as sand, resin-coated sand or sintered bauxite) which tend to erode the components of the plug valve that form the flow path through the valve. As is well known in the art, component wear causes close-fitting components to loose their initial shape, thereby rendering the plug valve more prone to leaking. In order to mitigate the effects of wear, plug valves used in severe service conditions require regular maintenance and refurbishment. The non-replaceable components of plug valves are typically refurbished by welding new metal in the flow path, and then machining the flow path components back to their original tolerances. Not only is refurbishment by welding and machining time-consuming and expensive, but the mechanical and thermal properties of the weld are inferior to those of the steel used to make the plug valve. Furthermore, for sour service, where welds are exposed to H2S, the welds are susceptible to sulphide stress corrosion cracking (SSCC).
Consequently, there is a need for a plug valve with improved erosion resistance that is more quickly and easily refurbished to original specifications.
A further common problem with high pressure plug valves is low pressure containment. Although plug valves generally provide a very reliable seal at high fluid pressures because in the closed position the high fluid pressures force the plug into a metal-to-metal sealing contact with the plug cage. However, when fluid pressures are low, especially with valves that have seen some severe service, the low pressure fluids may leak around the plug. This problem is most often experienced when setting up for a well stimulation treatment, or after flow back of the well before the pressure isolation equipment is removed from the wellhead. Leakage around the plug valve can permit hydrocarbons to escape to atmosphere, which is very undesirable.
There therefore also exists a need for a plug valve that provides a reliable seal at low fluid pressures.